1. Field of the Invention
The present invention relates to a suspending agent for suspension polymerization of a vinyl compound. More specifically, the present invention pertains to a suspending agent having an excellent less-deforming effect for suspension polymerization of a vinyl compound such as vinyl chloride.
2. Description of the Prior Art
In industrially producing a vinyl polymer such as a vinyl chloride resin, there has heretofore been widely practiced a suspension polymerization in which a vinyl compound such as vinyl chloride is suspended in an aqueous medium in the presence of a suspending agent and polymerized by the use of an oil-soluble catalyst.
In general, the factors regulating the quality of a vinyl polymer include conversion to polymer, water-monomer ratio, polymerization temperature, kind and amount of catalyst, type of polymerization vessel, agitation rate, kind and amount of suspending agent and the like. Among them, the kind of suspending agent has a large effect.
The performances of a suspending agent required for suspension polymerization of a vinyl compound are (1) a function of making the granulometric distribution of vinyl polymer granules obtained as sharp as possible; (2) a function of making each polymer granule porous in order to increase the absorption rate of a plasticizer for facilitating the workability, to facilitate the removal of monomers such as vinyl chloride residue in polymer granules and to prevent the formation of fish eye, etc. in the molding; and (3) a function of preparing polymer granules with a larger bulk density, and the like.
As a suspending agent for suspension polymerization of a vinyl compound, there has heretofore been used a cellulose derivative such as methyl cellulose or carboxymethyl cellulose or a partially hydrolyzed polyvinyl alcohol or the like, alone or in combination. However, the conventional suspending agents have a problem of no satisfying the required performances (1) to (3) described above.
The suspension polymerization of a vinyl compound such as vinyl chloride is generally carried out by batch production, where it is generally carried out by the method of charging an aqueous medium, a suspending agent, a polymerization initiator, a vinyl compound and the like in a polymerization vessel, further adding a necessary additive therein, and thereafter raising the temperature for polymerization.
Recently, it has been required to shorten the time period of one batch for polymerization in order to improve the productivity. In the suspension polymerization of a vinyl compound, therefore, the efficiency of removing the heat of polymerization has been improved by equipping a reflux condenser and the like; and a method of charging a preliminarily heated aqueous medium (hot-charge method) has been adopted for shortening the time period for the elevation of the temperature.
However, the conventional suspending agents for suspension polymerization of a vinyl compound has caused the the decrease of the productivity due to the decrease in the effective volume of a polymerization vessel and impossible temperature control in the case of the use of a polymerization vessel equipped with a reflux condenser because the conventional suspending agents give severe foaming during the polymerization; and the serious disadvantage of the decrease of the porosity of the vinyl polymer granules when the hot-charge method is used.
Alternatively, in order to prevent foaming, the addition of a deforming agent and the like decreases the thermal stability of the vinyl polymer granules, disadvantageously.
The Poval Book (issued by Polymer Press, 1984) (prior art (a)) describes, as a suspending agent for suspension polymerization of vinyl chloride, a polyvinyl alcohol having a degree of polymerization of 2,000 and a degree of hydrolysis of 80 mol %; and a polyvinyl alcohol having a degree of polymerization of 700 to 800 and a degree of hydrolysis of 70 mol %, respectively. WO 91/15518 (prior art (b)) describes a suspending agent for suspension polymerization of a vinyl compound comprising a polyvinyl alcohol having a degree of polymerization of at least 100, a degree of hydrolysis of 50 to 90 mol % and an amino group, an ammonium group, a carboxyl group or a sulfonic acid group at an end thereof. However, the suspending agents described in the prior arts (a) and (b) have a drawback to cause severe foaming during the polymerization.
Japanese Patent Application Laid-open No. 110797/1977 (prior art (c)) describes a secondary suspending agent for suspension polymerization of vinyl chloride comprising a polyvinyl acetate having a degree of hydrolysis of 30 to 60 mol %. Japanese Patent Application Laid-open No. 145208/1994 (prior art (d)) describes a secondary suspending agent for suspension polymerization of a vinyl compound comprising a water-insoluble polymer having a mercapto group at an end thereof and, as said water-insoluble polymer, exemplified by a polyvinyl ester polymer having a degree of hydrolysis of 50 mol % or less and a mercapto group at an end thereof. However, the secondary suspending agents described in the prior arts (c) and (d) cannot give a stable suspension polymerization of a vinyl compound, when said secondary suspending agents are used alone. Whereas the severe foaming is caused in the polymerization vessels when said secondary suspending agents are used in combination with the conventional partially hydrolyzed polyvinyl alcohol.
Polyvinyl Alcohol-Development (C. A. Finch, Wiley, 1992) (prior art (e)) describes an emulsion polymerization of methacrylate ester or styrene by the use of a polyvinyl alcohol having a degree of polymerization of 1,500, a degree of hydrolysis of 88 mol % and a mercapto group at an end thereof. However, the emulsion polymerization has a problem of being difficult to obtain powdered polymer being suitable for melt molding from the aqueous emulsion because a water-soluble initiator is used and the polymer obtained is in the form of an aqueous emulsion with a particle size of about 0.1 to 2 .mu.m in an emulsion polymerization described in the prior art (e).
It is an object of the present invention to provide a suspending agent having an excellent less-forming effect in a polymerization vessel and concurrently to satisfy the required performances (1) to (3) described above even in the case of a method of the use of a polymerization vessel equipped with a condenser, the hot-charge method and the hot-charge method with the use of a polymerization vessel equipped with a condenser, as well as in the case of the conventional general method for suspension polymerization of a vinyl compound of charging an aqueous medium at room temperature (cold-charge method) and the method of controlling the polymerization temperature by a jacket or a coil equipped to a polymerization vessel.